PROJECT DESCRIPTION Type III secretion systems (T3SSs) are widely distributed among gram-negative bacteria symbiotic or pathogenic to animals, plants and insects. These complex specialized machines have evolved to transfer multiple bacterial-encoded proteins, known as bacterial effectors, into host eukaryotic cells to modulate a variety of cellular functions, thus shaping the bacteria/host cell functional interphase for the benefit of the injecting bacteria. A central component of T3SSs is a supramolecular structure known as the needle complex (NC), which mediates the passage of the secreted proteins through the bacterial envelope. The functionality of the NC as a secretion machine is conferred by other components of the T3SS such as the export apparatus, which mediates the passage of the secreted proteins through the bacterial inner membrane, the sorting platform, a cytoplasmic structure that establishes the order in the secretion process, and an associated cytoplasmic ATPase that energizes the system. This research project focuses on the structural and functional characterization of the sorting platform. Through a multidisciplinary approach involving genetics, biochemistry, cryo-electron tomography, and super-resolution microscopy we will try to obtain structural and functional information on this essential component of T3SSs.